Frequency discriminator



May 15, 1945.

M. G. CROSBY FREQUENCY DISCRIMINATOR Filed April 27, 1943 2 Sheets-Sheet 1 CARRIER T1 :1 E.

09100 l/LATED Dsnonw Ou TPU T INVENTOR WZMA'MY 6. (kwax ATTORNEY n r p k V These various functions circuit depictedin Fig. 1. Here, the put trans former'lhas'prlmaryand -circuitsP.

Patented May 1 5, 1945 UNITED STATES PATENT orrlcg.

, 2.3mm FREQUENCY mscnnmva'ron Murray G. Crosby, Rlverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 27, 1943, Serial No. 484,704 6 Claims. (01. 250-40) phase modulated (PM hereinafter for brevity) carrier waves, or amplitude modulated (AM for brevity) carrier waves, for the purpose of providingfiltered carrier energy to be fed to a carrier-exalted demodulator, the filter network concurrently providing a vpair of complementary sloping filter characteristics for frequency discrimination.

The novel features which I believe to .be characteristic of my invention 'are set forth with particularity in theappended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, in whichI-have indicated diagrammatically several circuits Inthe drawings:

Fig. 1 shows an embodiment of the invention, Fig. 2 shows the frequency discrimination characteristic,

Fig. 3 shows a.

circuit, v

Fig. 4 illustrates a modification.

'I'hecrystal filter network of my present'invensimplified explanation of the and S each tuned to the mean, or center, frequency Fe of applied radio signals. Let it be assumed that the receiver is of the superhetero-.

dyne type, and that there is fed to the primary circuit P intermediate frequency (I. F.) energy. Let it further be assumed-that the receiver is a carrier 'exaltation receiver. This means that a parallel I. F. branch circuit Ill would be coupled to the network feeding P, and that the :branch circuit would feed a. demodulator ,l I. The latter would be fed with the substantially pure carrier output of the crystal X. For example, lead l2 feeds pure carrier energy. to. demodulator ll. Whether the signal energy is PM wave energy or AM wave energy, the same mode of operation would apply. Insofar as the AFC action is con-r cerned, in either of PM or AM reception employing carrier exaltation, it is'desirable to have a sharp AFC action so that the local oscillator is rigidly held in tune thereby to keepthe modulatedcarrier and filtered carrier at the demodulator in strict frequency synchronism.

The secondary circuit S has its coil S1 grounded at the midpoint thereof. The upper end of S1 connects to the anode of rectifier l, a diode, by piezo-electric crystal X. The latter is ground so as to be tuned to Fe, and is of the whereby my invention may be carried into effect.

type having one input electrode 3 and a pair of output electrodes 4 and 5. These electrodes may be conventional holder plates, or they may I be metallic coats provided on the opposite crystal faces. Electrode 4 connects to anode l of The anode 2' of diode rectifier 2 is connected to the second output electrode 5 of the tion functions, as set forth above, to provide substantially pure carrier energyfor carrier exaltation reception; to provide AFC (automatic frequency control) bias for the receiver local oscillator; and to provide a response to either-PM or AM wave energy. In other-words, the present crystal filter discriminatorwill function in either PM or AM receivers to provide sharp AFC action and carrier exaltation. Furthermore, the circuit provides two points at which the filtered or stripped carrier may be derived. Another feature of the invention is the fact that while using but 'a single crystal, yet the discriminator m y be designed to have two spaced ofl-centerifreare provided by thecrystal. The lower end of winding S1 feeds neutralizing voltage through adjustable condenser 8 to the electrode 5. This neutralizing voltage bucks out,ornullifies, the inter electrode capaci-- tance existing between electrodes 3 and; 5. The

condenser 9 performs the same function in con nection with the capacity'between electrodes 3 and 4. Since the midpoint of S is grounded, the voltage fed through condensers 8 and 9 necessarily is of opposite'phase'to that transmitted through capacitance 35 and 3-4 respectively.

AclJustable co1l-Lis arranged in series with crystal X to providethe off-center peak frequency F1. The, adjustable condenser C is arranged in series with the crystal to'provide the off-center peak frequency F2. In Fig. 2 there are .graphically indicated the off-center peak frequencies I Fr and F2, and. the manner in which the two sharp resonance curves cross over at Fe. It will be understood that curve a of Fig. 2 is the resonance curve of crystal'x in series with inductand I.

crystal X in series with condenser C. The magnitudes of L and C are so chosen as to provide a predetermined frequency spacingbetween F1 and F2. The frequencies F1 and F2 are equally spaced from Fe. Hence, it will be seen that there has been provided a frequency discriminator which presents at its output terminals complementary sloping filter characteristics.

The inductive reactance L feeds rectifier I. The anode If is connected to the upper end of coil L. The cathode of diode I is connected to the lower end of the coil by load resistor 6. The direct current return path 6' connects the anode and cathode of diode I directly together. The capacitative reactance C feeds the diode 2. The anode 2' is connected to the lower end of condenser C, while the cathode is connected to the junction of L and C by load resistor I. Each of load resistors 6 and 'I is bypassed by a respective I. F. bypass condenser. The load resistors are series-arranged, and the cathode end of resistor I is grounded, Return resistor I connects the anode 2' to the junction of resistors 6 The lower end of resistor 6 is, also, connected to the same junction'point. Furthermore, the junction of L and C is connected to the junction of :6 and 1'. It will be seen that diode I derives its signal voltage from across L, while diode 2 derives its signal voltage from across C.

The AFC bias is derived from the cathode end of loadresistor 6. The substantially pure or filtered carrier energy may be tapped from one of two points in the system. .It may be taken v signals.

The AFC bias production will be better. understood by reference to Fig. 3. Here the crystal X is depicted by its equivalent circuit. By the use of two output electrodes, two equivalent crys-' tal circuits X and X are provided. These circuits are series resonant circuits, tuned to Fe, which have a certain value of resistance. Coil L in series with the equivalent series resonant circuit X provides a total networktuned to F1.

Condenser C in series with the series resonant circuit X provides a network tuned to F2. Rectifier I is adapted for connection across L, while rectifier 2 is shunted across C. Hence, if ap plied signal energy has its mean frequency equal to Fe, then there will be equal signal voltage fed across each of L and C to their respective rectifiers. The latter being in opposition, the differential rectified voltage at the upper end of resistor 6 will be zero. L

Should the mean frequency of applied signal energy shift to F1, then more signal voltage will be developed across L. This follows from the fact that X in series with L tunes to F1.

produce equal and opposite reactive effects. For

instance, tuned circuits could be substituted for L and C, and the opposite reactive efiects produced by detuning'the tuned circuits in opposite directions from the frequency Fe. Fig. 4 shows how this may be done. In that figure the tuned circuit tuned to F1 replaces L, while the tuned circuit tuned to F2 replaces C. It will be noted that the return resistors 6 and I may be omitted in this modification.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a frequency discriminator circuit, a signal carrier input circuit, a piezo-electric crystal element provided with an input electrode and a pair of output electrodes, said crystal being tuned v to a desired reference frequency equal to'the frequency of said input circuit, means connecting the input electrode to one side of the input circuit, means grounding the electrical midpoint of the input circuit, means connecting one crystal output electrode to the opposite'side of the input circuit, an inductive impedance in series with one output electrode of the crystal and being resonant with the latter to a frequency spaced from said reference frequency, a capacitative impedance in series with the second output electrode of the crystal and being resonant with the latter to a frequency spaced from the reference frequency in the opposite sense, and respective output connections to each of said impedances.

2. In a frequency discriminator circuit, a signal carrier'input circuit, a piezo-electric crystal element provided with an input electrode and a pair of output electr0des,said crystal being tuned to a desired reference frequency, means grounding the electrical midpoint of the input circuit, means connecting said input electrode and one crystal output electrode to opposite sides of the electrical midpoint, an inductive reactance in series with one output electrode of the crystal and being resonant with the latter to a frequency spaced from said reference frequency, a capacitative reactance in series with the second output electrode of the crystal and being resonant with the latter to a frequency spaced from the reference frequency in the opposite sense, respective output connections to each of said reactances, and means grounding the junction of said reactances for signal currents.

3. In a frequency discriminator circuit, a signal carrier input circuit, a piezo-electric crystal element provided with an input electrode and a pair of output electrodes, said crystal being tuned to the frequency of said input circuit, means connecting the input electrode to one side of the input circuit, means grounding the electrical midpoint of the input circuit, means connecting one crystal output electrode to the opposite side of the input circuit, an inductive impedance in series with one output electrode of the crystal and being resonant with the latter to a frequency spaced from said reference frequency, a capacitative impedance in series with the second output electrode of thecrystal and being resonant with the latter to a frequency spaced from the reference frequency in the opposite sense, a respective rectifier connected to each of the impedances,

and the rectiflers being connected in opposition to provide a differential rectified voltage. v

4. In a frequency discriminator circuit, a signal carrier input circuit, apiezo-ele'ctric crystal element provided with an. input electrode and a pair of output electrodes, said crystal being tuned to the frequency of said input circuit, means connecting the input electrode to one side of the input circuit, means grounding the electrical midpoint of the input circuit, means connecting one crystal output electrode to the opposite side of the input circuit, an essentially inducl, tive impedance in series with one output electrode of the crystal and being resonant with the latter' to a frequency spaced from said reference frequency, essentially capacitative impedance in series with the second output electrode of the crystal and being resonant with the latter to a 'to the input circuit, a piezo-electric crystal element provided with an input electrode and a pair I of output electrodes, said crystal being tuned to a desired reference frequency, means grounding the electrical midpoint of the input circuit, means connecting said input electrode and one crystal output electrode to opposite sides of the electrical midpoint, means connecting the second output electrode to the side of the midpoint to which the one output electrode is connected, an inductive reactance in series with one output electrode of the crystal and tuning the crystal to a frequency spaced from said reference frequency, a capacitative reactance in series with the second output electrode of the crystal and being resonant with the latter to a frequency spaced from the reference frequency in the opposite sense, respective output connections to each of said reactances, means grounding the junction of said reactances for signal currents, and means for deriving substantially unmodulated signal carrier energy from at least one output electrode.

6. In a frequency discriminator circuit, a signal carrier input circuit, a piezo electric crystal element provided with an input electrode and a pair of output electrodes, said crystal being tuned to the frequency of said input circuit, means connecting the input electrode to one sided the input circuit, means grounding the electrical midpoint of the input circuit, means connecting one crystal output electrode to the opposite side of the input circuit, a tuned circuit in a series path with the crystal, means connecting one side of saidtuned circuitto one of'the crystal output electrodes, said tuned circuit being resonant to a frequency spaced from said reference frequency, a second tuned circuit in a series path with the crystal, means connecting one side of said second tuned circuit to the second crystal output electrode, said second tuned circuit being resonant to a frequency spaced from the ref .erence frequency in the opposite sense, means trode filtered-energy of substantially carrier frequency.

' MURRAY G. CROSBY. 

